Electron impact (EI) ionization is widely employed by mass spectrometry for environmental analysis and technological control. Samples of interest are extracted from analyzed media, like food, soil or water. The extracts contain analytes of interest within rich chemical matrixes. The extracts are separated in time within single or two-dimensional gas chromatography (GC or GC×GC). A GC carrier gas, typically Helium, delivers the sample into an EI source for ionization by an electron beam. Electron energy is generally kept at 70 eV in order to obtain standard fragment spectra. Spectra are collected using mass spectrometer and then submitted for comparison with a library of standard EI spectra for identification of analytes of interest.
Many applications demand analysis at high level of sensitivity (e.g., at least under 1 pg and preferably at 1 fg level) and with a high dynamic range (e.g., at least 1E+5 and desirably at 1E+8) concentrations between low level analytes and rich chemical matrix. Data with high resolving power is generally sought for reliable compound identification and for improving of signal to chemical noise ratio.
Many GC-mass spectrometer systems employ quadrupole analyzers. Since EI spectra contain a multiplicity of peaks, it is generally necessary to use a scan mass analyzer over a wide mass range, which leads to inevitable ion losses in quadrupole mass analyzers, slows down spectra acquisition, and introduces skew in the shape of individual mass traces, distorting fragment intensity ratios. Since GC and in particular GC×GC separation provide short chromatographic peaks (e.g., under 50 ms in GC×GC case), a Time-of-flight mass spectrometer (TOF MS) is generally used for rapid acquisition of panoramic (full mass range) spectra when coupled with GC or GC×GC